The researchers, led by Junjiu Huang, a gene-function researcher at Sun Yat-sen University in Guangzhou, used a technology called CRISPR to find bad sections of DNA in the embryos, cut them out, and replace them.

To do this, they injected the embryos with the enzyme complex CRISPR/Cas9. The complex can be programmed to target a problematic gene, then bind and splice the DNA at specific locations, replacing or repairing that gene.

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The team attempted to modify the gene responsible for β-thalassaemia, a potentially fatal blood disorder. They injected 86 “non-viable” embryos–meaning they could not result in a live birth–then waited 48 hours to allow CRISPR to act and the embryos to grow to about eight cells each.

Seventy-one of the embryos survived, and 54 were genetically tested. Of those 54, only 28 were successfully spliced; and only “a fraction” of those contained the replacement genetic material, prompting the research team to stop their human testing. “If you want to do it in normal embryos, you need to be close to 100%,” Huang said. “That’s why we stopped. We still think it’s too immature.”

Reactions to the researchers’ experimentation have been mixed. Supporters laud the ability to fix potentially life-threatening genetic diseases such as cystic fibrosis, sickle cell anemia, and Huntington’s disease. Luhan Yang, a postdoctoral scientist at Harvard Medical School, said it may also be possible to install genes that would offer lifelong protection against infection, Alzheimer’s, and possibly even the effects of aging, leading Antonio Regalado to write for the MIT Technology Review that “[s]uch history-making medical advances could be as important to this century as vaccines were to the last.”

Others, though, insist editing the human genome is ethically questionable. There is concern from other scientists that changing the DNA of viable embryos could have unpredictable consequences for future generations, as the genetic changes, known as germline modifications, are heritable–meaning the changes would be passed down to offspring.

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Geneticist Dana Carroll of the University of Utah warned, “Germline genome alterations are permanent and heritable, so very, very careful consideration needs to be taken in advance of such applications.”

Others fear this technology could be used down the road to create “designer humans,” with features and characteristics specifically chosen and changed in the embryonic phase of development.

The debate over genetic engineering is one that is sure to continue for some time. Many countries, not including the U.S., have outlawed germline gene editing; and the European Union’s convention on human rights and biomedicine stated that tampering with the gene pool would be a crime against “human dignity” and human rights.

Do you think the ability to modify the human genome is something that should be utilized, or are these scientists trying to “play God”?